Method for treating combustion ashes

ABSTRACT

A method is provided for treating ashes from the combustion of petroleum fuels by mixing the combustion ashes with water to produce a slurry; injecting either calcium oxide or calcium hydroxide into the slurry and reacting the calcium oxide or calcium hydroxide with the ammonium sulfate within the ashes to produce a slurry containing gypsum and ammonia; flowing the slurry containing gypsum and ammonia down from the top portion of packed column and air or steam up from the bottom portion of the packed column so that the air or steam strips the ammonia from the gypsum slurry; and separating the gypsum from the resulting ammonia-free slurry using a solid/liquid separator.

BACKGROUND OF THE INVENTION

The present invention relates to a wet-processing method for combustionashes of petroleum fuels, containing ammonium sulfate and a method ofutilizing ammonia components recovered by the wet-processing method.More in particularly the present invention relates to a wet-processingmethod which comprises at least a preparing-step of a slurry ofcombustion ashes of mixing combustion ashes and water, a doubledecomposition step for ammonium sulfate, a recovering-step of freeammonia from a gypsum slurry containing free ammonia produced in thedouble decomposition step, and a gypsum separation step successively,which is excellent in an ammonia stripping efficiency from the gypsumslurry in the ammonia recovery step and in which the clogging in astripping device is suppressed, as well as a method of utilizing ammoniacomponents with an industrial advantage.

In various kinds of combustion furnaces (combustion devices) usingpetroleum fuels such as heavy oils and or emulsions, etc., for example,boilers of oil fired power stations and dust incinerators, ammonia isadded in the combustion gases for preventing the corrosion of thecombustion furnaces, caused by sulfate gases (S03) contained incombustion gases.

Accordingly, the combustion ashes collected and recovered by anelectrostatic precipitator disposed to the downstream of a fire flue endthereof contain ammonium sulfate in addition to ashes mainly composed ofunburnt carbon and heavy metals (Ni, V, Mg, etc.). For example, thefollowing ingredients are contained in the combustion ashes recoveredfrom a boiler using high sulfur-content heavy oils.

C: 10 to 80% by weight

NH₄ : 0.5 to 20% by weight

SO₄ : 20 to 60% by weight

V: 1 to 5% by weight

Ni: 0.3 to 2% by weight

Fe: 0.3 to 2% by weight

Mg: 0.1 to 8% by weight

SiO₂ : about 0.1% by weight

As a method of treating the combustion ashes, a lot of methods, referredto as a wet-process, for recovering valuable ingredients such asvanadium, which take a countermeasure for public pollution by a closedsystem, have been proposed. Specifically, there can be mentionedwet-processes, for example, as disclosed in Japanese Patent ApplicationLaid-Open (KOKAI) Nos. 60-19086 and 60-46930, and Japanese PatentPublication (KOKOKU) Nos. 4-61709 and 5-13718 already proposed.

For example, the process described in Japanese Patent Publication(KOKAI) No. 5-13718 comprises (i) a first step of mixing combustionashes and water to form a slurry, while optionally adding sulfuric acidto control the pH value to not more than 3, (ii) a second step ofseparating solids (unburnt carbon, etc.), (iii) a third step of heatingthe obtained liquid portion to a temperature of not less than 70° C. andoxidizing metals while supplying ammonia and an oxidant to adjust the pHvalue to 7 to 8, (iv) a fourth step of separating deposits (ironsludges), (v) a fifth step of cooling the obtained liquid portion to atemperature of 40° C., thereby depositing vanadium compounds (ammoniummetavanadate), (vi) a sixth step of separating the deposited vanadiumcompounds, (vii) a seventh a step of adding calcium hydroxide or calciumoxide to the obtained liquid portion to deposit gypsum and metal (nickeland magnesium) hydroxides, and liberating ammonia simultaneously, (viii)an eighth step of recovering ammonia by stripping from free ammonia, and(ix) a ninth step of separating the obtained gypsum.

However, there are drawbacks in the ammonia recovery methods describedin the above-mentioned publications, respectively.

Namely, the method disclosed in Japanese Patent Application Laid-Open(KOKAI) No. 60-19086 is a method comprising complicated steps ofprecipitating gypsum from a slurry before stripping, and passing theresultant supernatant through a distillation column to strip an ammoniagas. Such a method is industrially disadvantageous, in which calciumcompounds such as gypsum and magnesium hydroxide tend to be deposited onthe inner wall surface of the distillation column and the operationthereof can not be continued for a long time.

Further, the method described in Japanese Patent Application Laid-Open(KOKAI) No. 60-46930 is a method comprising supplying steams from thelower portion of an aerating tank so as to heat to a temperature of notless than 80° C. and supplying air from the lower portion thereof so asto strip ammonia. In this method, the stripping efficiency is poorbecause the contact between the slurry and air is insufficient, andcontinuous operation is difficult.

Further, the methods described in Japanese Patent Publication (KOKOKU)Nos. 4-61709 and 5-13718 comprise supplying a slurry containing a largeamount of gypsum to a separation column from the upper portion thereofbut no concrete example for the separation column is described.

Referring to the recovered ammonia, it is supplied to a fire flue of aboiler or a metal oxidation step for reutilization. However, sinceammonia separated from each of the processes described above is a gasousmixture containing a water content (moisture) evaporated from the slurryand a large amount of air, pipelines of a large diameter are requiredfor transporting to a place to be utilized. Further, since the watercontent contained therein is condensed into drains, a draining devicehas to be disposed at the midway of long distance pipelines, which isdisadvantageous in view of maintenance.

As a result of the present inventors' earnest studies, it has been foundthat in a recovering-ammonia step of a wet-processing method forcombustion ashes of petroleum fuels, by flowing down a gypsum slurryfrom the upper portion of a packed column and blowing air or stream fromthe lower portion thereof so as to bring air or stream intocounter-current contact with the gypsum slurry, ammonia is stripped fromthe gypsum slurry, an ammonia stripping efficiency from the gypsumslurry in the ammonia recovery step of the wet-processing method isexcellent and the clogging in the packed column is suppressed, that is,though it has been considered that the packed column can not be used fora slurry containing a large amount of solids because of a cloggingproblem since the packed column contains a large amount of packings andhas a narrowed channel, the packed column in the wet-processing methodcan be used with no practical problem. On the basis of theabove-mentioned finding, the present invention has been attained.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a wet-processing methodfor combustion ashes of petroleum fuels, containing at least ammoniumsulfate and vanadium, which comprises a preparing-step of a slurry ofcombustion ashes, obtained by mixing combustion ashes and water; adouble decomposition step for ammonium sulfate; a recovering-step offree ammonia from a gypsum slurry containing free ammonia produced inthe double decomposition step; and a gypsum separation stepsuccessively, which is excellent in an ammonia stripping efficiency fromthe gypsum slurry in the ammonia recovery step and in which clogging ina stripping device (for example, packed column) is suppressed.

Another object of the present invention is to provide an improved methodof utilizing ammonia components at a high efficiency, which comprisesefficiently transporting the thus obtained ammonia and supplying ammoniainto an exhaust gas channel of a combustion furnace, etc.

To accomplish the aims stated above, in a first aspect of the presentinvention, there is provided a wet-processing method for combustionashes of petroleum fuels, collected by a dust collector disposed in afire flue of an exhaust gas for a boiler or the like using petroleumfuels and containing at least ammonium sulfate and vanadium, whichmethod comprises:

at least

a preparing-step of a slurry of combustion ashes by mixing thecombustion ashes with water,

a double decomposition step for ammonium sulfate,

a recovering-step of free ammonia from a gypsum slurry containing freeammonia obtained in the double decomposition step,

the gypsum slurry being flowed down from an upper portion of a packedcolumn and air or steams being blown from a lower portion of the packedcolumn to bring air or steams into counter-current contact with thegypsum slurry, thereby conducting stripping for ammonia, and

a gypsum separation step.

In a second aspect of the present invention, there is provided a methodof utilizing an ammonia component recovered by the wet-processing methodas defined in the first aspect, which method comprises:

transporting a mixture of ammonia stripped in the ammonia recovery stepand steams in the form of an aqueous solution to an ammonia separationdevice disposed in a battery limit of an ammonia supply device in anexhaust gas channel of a combustion furnace;

mixing a gaseous ammonia separated and recovered by the ammoniaseparation device with air;

heating the mixture; and

supplying the resultant mixture into the exhaust gas channel of thecombustion furnace.

In a third aspect of the present invention, there is provided a methodof utilizing the ammonia component recovered by the wet-processingmethod as defined in the first aspect, for the preparation of ammoniummetavanadate, which method comprises:

a solid-liquid separation step of removing solids from the slurry of thecombustion ashes,

a metal oxidation step of supplying ammonia and an oxidative gas to anaqueous solution formed by removing the solids from the slurry of thecombustion ashes to oxidize vanadium, thereby obtaining an aqueoussolution containing ammonium metavanadate, and

a crystallization step for the obtained ammonium metavanadate,

the above-mentioned three steps being incorporated between thepreparing-step of the slurry of the combustion ashes and the doubledecomposition step of ammonium sulfate as defined in the first aspect,and

a mixture of ammonia stripped in the ammonia recovery step and steamsbeing transported in the form of an aqueous solution to the metaloxidation step.

In a fourth aspect of the present invention, there is provided a methodof utilizing the ammonia component recovered by the wet-processingmethod as defined in the first aspect for the preparation of ammoniummetavanadate, which method comprises:

a metal oxidation step of supplying ammonia and an oxidative gas to theslurry of combustion ashes to oxidize vanadium, thereby obtaining aslurry containing ammonium metavanadate,

a solid/liquid separation step of removing solids from the slurrycontaining ammonium metavanadate, and

a crystallization step of the obtained ammonium metavanadate,

the above-mentioned three steps being incorporated between thepreparing-step of the slurry of the combustion ashes and the doubledecomposition step as defined in the first aspect,

a mixture of ammonia stripped in the ammonia recovery step and steamsbeing transported in the form of an aqueous solution and supplied to themetal oxidation step.

In a fifth aspect of the present invention, there is provided a methodwherein in the method as defined in the third or fourth aspect, anoxidation vessel in which a gas supply pipe is disposed to protrude in areaction material is used, and heated steams are introduced togetherwith the oxidizing gas from the gas supply pipe.

In a sixth aspect of the present invention, there is provided a methodwherein in the method as defined in the third or fourth aspect, thereaction in the metal oxidation step is taken place at a hightemperature condition of not less than 100° C. under a pressure toobtain an aqueous solution in which the concentration of ammoniummetavanadate is not more than the saturation concentration at thetemperature of the aqueous solution and not less than 1.2% by weight,and the concentration of an ammonium sulfate is from 5 to 30% by weight,and

the obtained aqueous solution is transported under pressure and atemperature retaining condition through the supply pipe and supplied toa crystallization vessel in the crystallization step.

In a seventh aspect of the present invention, there is provided a methodwherein, in the method as defined in the fourth or fifth aspect, acontinuous crystallization method of cooling and crystallization anaqueous solution containing ammonium metavanadate supplied at a constantrate continuously to the crystallization vessel is used in thecrystallization step,

a crystallization device comprising the crystallization vessel and acooler disposed to the outside thereof which are connected by means of acirculation channel is used,

a slurry in the crystallization vessel in an amount more than that ofthe raw material solution supplied is circulated to the cooler, and

a slurry substantially identical in the amount with that of the rawmaterial solution supplied is withdrawn from the crystallization device.

In an eighth aspect of the present invention, there is provided a methodwherein in the method as defined in the first aspect, two units ofsolid/liquid separators are used in series in the gypsum separationstep, with the first solid/liquid separator being a centrifugalprecipitation-type solid/liquid separator and the final solid/liquidseparator being a centrifugal precipitation-type solid/liquid separatoror a filtration type solid/liquid separator, and

which method comprises:

processing a gypsum-containing slurry (A) by the first solid/liquidseparator,

separating into gypsum (C) and a supernatant (B) containing nickeland/or magnesium hydroxide mixed from the gypsum-containing slurry (A)and containing 1 to 20% by weight of gypsum, calculated as solids, basedon the hydroxides, and

processing the supernatant (B) by the final solid/liquid separator toseparate into a solid (D) and a waste water (E) not substantiallycontaining the solid.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is explained specifically herein.

Description is made at first for a wet-processing method for combustionashes of petroleum fuels, containing ammonium sulfate, that is, a methodof recovering ammonium components from the combustion ashes of petroleumfuels.

Combustion ashes as an objective to be processed in the presentinvention are combustion ashes containing at least ammonium sulfate andvanadium, which are collected by a dust collector disposed, for example,at the end of an exhaust gas flue of a boiler using petroleum fuels.Such combustion ashes are obtained by being collected in anelectrostatic precipitator of various kinds of combustion furnaces(combustion devices) operated with addition of ammonia to combustiongases.

The wet-process in the present invention comprises at least apreparing-step of a slurry of combustion ashes by mixing combustionashes with water, a double decomposition step for ammonium sulfate, anda recovery recovering-step of free ammonia from a gypsum slurrycontaining free ammonia (free ammonia-containing gypsum slurry) obtainedin the double decomposition step and a gypsum separation stepsuccessively, to which optional steps may be added so long as theabove-mentioned steps are included.

In a preferred embodiment of the present invention, that is, in a methodof utilizing ammonia components for producing ammonium metavanadate,steps of the following process (1) or (2) are conducted between thepreparing-step of the slurry of the combustion ashes and the doubledecomposing step for ammonium sulfate.

(1) A solid/liquid separation step of removing solids from the slurry ofthe combustion ashes; a metal oxidation step of supplying ammonia and anoxidative gas to an aqueous solution obtained by removing solids fromthe slurry of combustion ashes to oxidize vanadium, thereby obtaining anaqueous solution containing ammonium metavanadate; and a crystallizationstep of the thus obtained ammonium metavanadate are disposed,successively.

(2) A metal oxidation step of supplying ammonia and an oxidative gas tothe slurry of combustion ashes to oxidize vanadium, thereby obtaining aslurry containing ammonium metavanadate; a solid/liquid separation stepof removing solids from a slurry containing ammonium metavanadate; and acrystallization step of the thus obtained ammonium metavanadate,successively.

The preparing-step of the slurry of combustion ashes is, for example, amethod of mixing combustion ashes of petroleum fuels and water, addingsulfuric acid to the mixture to adjust the pH value to 1.5 to 3, heatingthe resultant mixture to a temperature of 40 to 70° C., therebyconverting metals such as vanadium into ions, and dissolving ammonia asammonium sulfate.

In the solid/liquid separation step of the above-mentioned processes (1)and (2), known methods using, for example, a centrifugal separator orfilter press are adopted. The metal oxidation step and crystallizationstep in these processes will be described later.

In the double decomposition step of ammonium sulfate, calcium hydroxideor calcium oxide is used as a strong base. The strong base is usedusually in a stoichiometrical or slightly excess amount to sulfateradicals in the gypsum to be formed. By the double decomposition, thefree ammonia-containing gypsum slurry is formed. Due to the presence offree ammonia and unreacted calcium hydroxide or the like, the pH valueof the gypsum slurry is usually from 9 to 12, preferably from 10 to 11.

In the ammonia recovery step, a counter-current type packed column whichis most excellent in the stripping efficiency is used. Then, freeammonia contained in the gypsum slurry is stripped. When a plate columnis used for striping free ammonia, because the slurry is deposited toeach of the trays, continuous operation for the stripping of the gypsumslurry is difficult. Further, since a wetted wall column has no trays asin the plate column and contains no packings, it suffers less cloggingeven in a case of a gypsum slurry containing a large amount of solids,but the area of contact between the gypsum slurry and thecounter-current gas is small, so that the stripping efficiency for freeammonia is low.

The inside of the counter-current type packed column is structured suchthat packings with large surface area such as Raschig rings, Lessingrings and saddle-type packings are packed in a large amount. Then, thegypsum slurry flows down along the surface of the packings. Accordingly,the counter-current type packed column has a large area of contactbetween the gypsum slurry and the counter-current gas, and is excellentin the efficiency of stripping free ammonia.

The pH value of the gypsum slurry supplied to the packed column isadjusted to preferably not less than 11, more preferably not less than12. If the pH value is less than 11, the amount of magnesium hydroxidedissolved in the slurry may be increased, and as a result, insolubilizedmagnesium hydroxide may be deposited, for example, on the surface of thepackings in the packed column, resulting in a worry of clogging a slurrychannel in a case of long time operation.

Further, the concentration of gypsum in the gypsum slurry is adjusted topreferably from 7 to 40% by weight, more preferably from 10 to 30% byweight. If the gypsum concentration is less than 7% by weight, gypsummay tend to be deposited on the inner wall surface of the packed columnand the surface of the packings, so that particularly, during long timeoperation, there is a worry that the gypsum slurry channel may beclogged by the deposition of gypsum. On the other hand, if the gypsumconcentration exceeds 40% by weight, since the viscosity of the gypsumslurry may be large, there is a worry that the narrow flow channel uponflowing down in the packed column may be clogged.

In a case of supplying a supernatant obtained by precipitating of gypsumfrom the gypsum slurry (aqueous saturated solution of gypsum) to thepacked column, when the gypsum is crystallized due to the effect oftemperature change or the like, since the solid (gypsum) to whichcrystallized gypsum is deposited is not present in the solution, thisresults in deposition of gypsum on the inner wall surface of the packedcolumn and the surface of the packings.

There is no particular restriction for the temperature of the gypsumslurry supplied to the packed column, but it is preferably heated forpromoting stripping of ammonia. Such a heating temperature is usuallyfrom 70 to 110° C., preferably from 90 to 105° C.

As a counter-current gas supplied to the packed column, air and steammay be used. The steam is preferred and further, heated steam is morepreferred. The temperature of the steam is usually from 110 to 190° C.,preferably from 130 to 160° C. In a case of using the steam as acounter-current gas, since ammonia is separated in the form of a mixturewith steam and dissolved into the water content of the steam, it is inthe form of an aqueous solution after cooling.

Accordingly, when recovered ammonia is stored or transported to othersteps or the devices, the diameter of transportation pipelines can bedecreased efficiently, and further, it can be sprayed as it is, whenadded to the fire flue described later, which is suitable also in viewof use.

In a case of using air as the counter-current gas, it is separated as agaseous mixture of air, ammonia and steam evaporated from the gypsumslurry. In this case, the gaseous mixture is converted into an aqueoussolution, for example, by a method of bubbling into water.

Then, a method of utilizing the ammonia components recovered by thewet-processing method described above is described.

In the first utilizing method of the present invention, ammoniarecovered by the wet-process is used as a neutralizing agent for asulfate gas (SO₃) contained in exhaust gases of a combustion furnace.Specifically, a mixture of ammonia and steam stripped in the ammoniarecovery step is transported in a state of an aqueous solution to anammonia separation device disposed in a battery limit of an ammoniasupply device in an exhaust gas channel of a combustion furnace, andthen gaseous ammonia separated and recovered by the ammonia separationdevice is mixed with air, heated and then supplied to the exhaust gaschannel of the combustion furnace. In this case, since ammonia is in thestate of an aqueous solution, no draining device is necessary to atransportation pipeline which is required for transporting in a gaseousstate, as well as the inner diameter of the transportation pipelines canbe reduced since the volume of ammonia is reduced. Accordingly, thefirst utilization method of the present invention is efficient both inthe size reduction of and management for the facility.

As the ammonia separation device, various types of known separationcolumns can be used. In this case, there can be mentioned a method ofsupplying aqueous ammonia to the separation column and bringing air orsteam as a separation medium into counter-current contact therewith. Forthe counter-current gas, steam is preferred in view of the separationefficiency.

There is no particular restriction for the ammonia content in thegaseous mixture of ammonia and air, and it is usually from 0.1 to 10% byweight, preferably from 0.5 to 5% by weight. The temperature for heatingthe gaseous mixture of ammonia and air is usually from 100 to 200° C.,preferably from 120 to 180° C. The gaseous mixture of ammonia and airmay also contain water. In this case, the water content is preferablynot more than 10% by weight based on the weight of the entire gaseousmixture.

As a method of supplying the gaseous mixture of ammonia and air in theexhaust gas channel of the combustion furnace, there can be mentioned,for example, a method of arranging a number of spray nozzles on the wallsurface in the channel or in a cross section of the channel, and jettingthe gaseous mixture from the nozzle since the gaseous mixture is sprayedas uniformly as possible over the cross section in the exhaust gaschannel.

The amount of the gaseous mixture containing ammonia and air isdetermined depending on the amount of exhaust gases and theconcentration of the sulfate gas in the exhaust gases, and usually, itis controlled such that ammonia in the gaseous mixture is in astoichiometrical or slightly excessive amount relative to the sulfategas in the exhaust gases.

By the way, there has been known a method of processing an exhaust gasof a combustion chamber by supplying ammonia into the exhaust gas of acombustion furnace at a temperature, for example, of usually not lessthan 700° C., thereby reducing nitrogen oxides in the exhaust gas of thecombustion chamber in a non-toxic state (refer, for example, to JapanesePatent Publication (KOKOKU) No. 56-40628).

In the method of the present invention, recovered ammonia can also beused as a reducing agent for the nitrogen oxides as described above.Known conditions can be applied as they are to processing conditionssuch as the amount of ammonia supplied to the exhaust gases of thecombustion furnace. In a case of using ammonia as the reducing agent fornitrogen oxides, since ammonia is consumed, ammonia has to be addedseparately as a neutralizing agent for sulfate gas (SO₃)

In the second utilization method of the present invention, ammoniarecovered by the wet-process is used for the production of ammoniummetavanadate. That is, a mixture of ammonia and steam stripped in theammonia recovery step is transported in the form of an aqueous solutionand supplied to the metal oxidation step of (1) or (2) described above.

In this process (1), a solid/liquid separation step--metal oxidationstep--crystallization step for ammonium metavanadate are conductedbetween the preparing-step of the slurry of combustion ashes and thedouble decomposition step of ammonium sulfate.

In the solid/liquid separation step, the solids (mainly, unburnt carbonand undissolved other components) are removed from the slurry of thecombustion ashes to obtain an aqueous solution.

In the metal oxidation step, ammonia and an oxidative gas are suppliedto the aqueous solution obtained by removing solids from a slurry ofcombustion ashes, thereby oxidizing vanadium, and an aqueous solutioncontaining ammonium metavanadate is prepared.

The pH value of the aqueous solution is adjusted to usually not lessthan 7, preferably 7 to 9. As method of adjusting the pH value of theaqueous solution to not less than 7, there can be mentioned, forexample, a method of supplying ammonium or ammonium compound whichserves also as a reactant. For adjusting the pH value, in the case wherethe ammonia component is present in a sufficiently stoichiometricalamount, caustic alkali such as caustic soda may be jointly used only forthe purpose of adjusting the pH value, it is preferred to adjust the pHvalue only by ammonia or ammonia compound in order to avoid introductionof additional chemical species into the reaction system.

As the ammonia component, ammonia recovered in the ammonia separationstep of the wet-process as described above is used in the form of anaqueous solution. If necessary, pure ammonia produced separately may beused additionally in the form of an aqueous solution. As the oxidativegas, there can be mentioned, for example, air, oxygen and ozone. Amongthem, air is preferred in view of practicality.

By the way, for the oxidizing reaction described above, an oxidationvessel in which a gas supply pipe is disposed so as to protrude into anaqueous solution in which vanadium and ammonium are dissolved, butammonium metavanadate is deposited as scales on the surface of openingsof the oxidative gas supply pipe, and in an extreme case, there iscaused a trouble such as clogging in the opening of the oxidative gassupply pipe.

Accordingly, in order to solve the foregoing problem, it is preferred touse an oxidization vessel in which the gas supply pipe is disposed so asto be protruded into a reaction material (aqueous solution in whichvanadium and ammonium are dissolved) and to introduce heated steamtogether with the oxidative gas from the gas supply pipe.

As described above, by the introduction of the heated steam togetherwith introduction of the oxidative gas, the concentration of ammoniummetavanadate at the opening of the gas supply pipe is diluted, and thetemperature lowering on the surface of the opening is suppressed. As aresult, the deposition of scales of ammonium metavanadate to the surfaceof the opening of the gas supply pipe can be prevented. Further, thescaling to a stirring blade disposed near a position above the openingof the gas supply pipe can also be suppressed.

The temperature of the heated steam is usually not less than 100° C. (1atm), and in a case of reaction in an increased inner pressure, it ispreferably not less than 120° C. (about 2 atm), more preferably not lessthan 135° C. (about 3 atm), even more preferably not less than 155° C.(about 5 atm). The amount of the heated steam to be introduced isusually controlled to such an extent as compensating the caloriecorresponding to the lowering of the liquid temperature by theintroduction of air.

Preferably, the oxidizing reaction is conducted under a pressure at ahigh temperature condition of not less than 100° C. so as to obtain anaqueous solution in which the ammonium concentration is not more thanthe saturated concentration at the temperature of the aqueous solutionand not less than 1.2% by weight, and the concentration of ammoniumsulfate is from 5 to 30% by weight. This is, because the solubility ofammonium metavanadate is increased unexpectedly along with an increasein the liquid temperature and the miniaturization of the device can beattained markedly. A more preferred lower limit for the reactiontemperature is 120° C. The upper limit of the temperature is usually150° C., preferably 135° C.

In the crystallization step, ammonium metavanadate is crystallized andrecovered from an aqueous solution containing ammonium metavanadateobtained in the metal oxidation step. Specifically, ammoniummetavanadate is crystallized by cooling a raw material solution suppliedcontinuously at a constant rate to the crystallization vessel.

By the way, in a case of using a crystallization vessel having interiorcooling coils, trouble may occur when crystals of precipitated ammoniummetavanadate are deposited on the surface of the cooling coils to lowerthe cooling performance. Accordingly, in order to solve the foregoingproblem, it is preferred to use a crystallization device comprising acrystallization vessel and a cooler disposed to the outside thereof,which are connected by a circulation channel.

It is preferred to circulate to the cooler a slurry in thecrystallization vessel in an amount more than that of the raw materialsolution supplied and withdraw from the crystallization device theslurry of an amount substantially equal with that of the raw materialsolution supplied.

If the slurry in the crystallization vessel is circulated to the coolerin an amount less than that of the raw material solution supplied, it isnecessary to take a large difference of the cooling temperature for theslurry in the cooler, and as a result, crystals of ammonium metavanadateare deposited in the cooler and the circulation channel. On thecontrary, when the slurry in the crystallization vessel is circulated tothe cooler in an amount more than that of the raw material solutionsupplied, the difference of the cooling temperature for the slurry inthe cooler can be reduced.

The amount of the slurry circulated to the cooler and/or the amount ofthe raw material solution supplied to the crystallization vessel ispreferably controlled such that the temperature lowering for the slurryby the cooler is not more than 5° C., and the temperature differencebetween the raw material solution supplied to the crystallization vesseland the slurry in the crystallization vessel is not less than 30° C. Thetemperature in the crystallization vessel is set to usually not morethan 40° C., preferably from 20 to 30° C.

For the method of recovering ammonium metavanadate, a method ofsedimentation and concentration by the use of a centrifugal separator,followed by cake filtration is usually adopted.

When solids such as iron sludges mainly comprising iron oxidesby-produced in the metal oxidation step are contained in the aqueoussolution of raw material before crystallization, the solids arepreferably removed before the crystallization step. By such procedures,ammonium metavanadate at high purity can be isolated.

Then, the process (2) is to be explained. In this process, metaloxidation step--solid/liquid separation step--ammonium metavanadatecrystallization step are conducted between the preparing-step of theslurry of the combustion ashes and the double decomposition step forammonium sulfate.

The process (2) is identical with the process (1) except that thesolid/liquid separation step is not conducted at first, but ammonia andoxidative gas are supplied to the slurry of the combustion ashes,thereby converting vanadium into ammonium metavanadate (metal oxidationstep), and solids are separated and removed from the resultant slurry ofthe combustion ashes containing ammonium metavanadate (solid/liquidseparation step).

In the solid/liquid separation step, unburnt carbon and iron sludge,etc. by-produced in the metal oxidation step are separated and removedtogether. In this case, since the pH value in the slurry of thecombustion ashes is in an alkaline region, the device used forseparation and removal can be suppressed from corrosion.

Accordingly, the process (2) is excellent as compared with the process(1) in that ammonium metavanadate at high purity can be isolated withoutdisposing the solid/liquid separation step for iron sludges.

The gypsum slurry remained after stripping of free ammonia in theammonia recovery step is supplied to the gypsum separation step.

By the way, Japanese Patent Publication (KOKOKU) No. 5-13718, forexample, describes that a centrifugal precipitating solid/liquidseparation device is suitable for the separation of gypsum, and thecentrifugal precipitation-type solid/liquid separation device issuitable for the separation of nickel sludge (for example, nickelhydroxide), since a filtration-type solid/liquid separation device tendsto suffer from clogging.

On the other hand, since gypsum described above can be recovered in arelatively large amount, it can be used in various application uses byseparating nickel and/or magnesium hydroxides to refine to a highpurity. The purity and the recovery amount of gypsum can be controlledto a desired range by properly selecting the operation conditions of thecentrifugal precipitating solid/liquid separation device. For example,in the solid/liquid separation device, the purity of gypsum can beimproved by lowering the recovery percentage of gypsum (increasing thegypsum concentration in supernatant).

However, as is well-known, since both of nickel hydroxide and magnesiumhydroxide are present as fine particles in a liquid, the clogging tendsto occur in the filtration-type solid/liquid separation device in a caseof separating such hydroxides from the supernatant as described in theabove-mentioned publication. Further, also in the centrifugalprecipitation-type solid/liquid separation device, the separation ofnickel and/or magnesium hydroxides is not always easy. As a result,there is a worry in the existent wet-processes that solids are containedin the drains and there is a drawback not capable of completelyattaining a countermeasure for public pollution by a closed system.

Then, for efficiently conducting the solid/liquid separation of nickeland/or magnesium hydroxides which are difficult to be separated, it ispreferred to adapt a system of using two units of the solid/liquidseparators in series, as the first solid/liquid separator, a centrifugalprecipitation-type solid/liquid separator is used and as the last(final) solid/liquid separator, a centrifugal precipitation-typesolid/liquid separator or a filtration-type solid/liquid separator isused. The gypsum-containing slurry (A) is treated by the firstsolid/liquid separator to separate the gypsum-containing slurry (A) intogypsum (C) and a supernatant (B) containing nickel and/or magnesiumhydroxides mixed from the gypsum-containing slurry (A) and containing 1to 20% by weight based on the weight of the said hydroxides. Then, thesupernatant (B) is treated by the last solid/liquid separator toseparate the same into a solid (D) and a waste water (E) containing nosubstantial solid.

The above-mentioned method has a mixing-step of a predetermined amountof gypsum in the supernatant (B) obtained from the first solid/liquidseparator.

As a result, when the centrifugal precipitation-type solid/liquidseparator (horizontal continuous decanter or the like) is used for thelast solid/liquid separator, the precipitation of nickel and/ormagnesium hydroxides is promoted by the entraining precipitating effectof gypsum having more excellent precipitating property than that ofnickel and/or magnesium hydroxide. Further, when the filtration-typesolid/liquid separator (filter press) is used for the last solid/liquidseparator, the clogging on the press surface can be suppressedeffectively by the coating on the press surface of gypsum havingexcellent filtration property than that of nickel and/or magnesiumhydroxide.

The adjustment for the amount of gypsum contained in the supernatant(B), in a case of using a horizontal-type continuous decanter as thecentrifugal precipitation-type solid/liquid separator, is conducted bychanging, for example, the feed rate of the slurry (A) to the decanterand the height of a dam disposed to the discharging portion of thesupernatant. A preferred mixing amount of gypsum in the supernatant (B)is 3 to 10% by weight by the same reason of the definition describedabove. Further, as the last solid/liquid separator, use of thefiltration-type solid/liquid separator (filter press) is preferred witha view point that the solid concentration in the waste water (E) is low.

According to the present invention described above, the stripping effectfor ammonia from the gypsum slurry is excellent and the clogging of thestripping device can be suppressed in the recovering-step of ammonia inthe wet-process described above for combustion ashes, and the method ofthe present invention is of a significant industrial worth.

EXAMPLES

The present invention will be explained more specifically by way ofexamples but the invention is not restricted only to the followingexamples, unless it does not exceed the gist thereof.

Example 1

As the combustion ashes of petroleum fuels, combustion ashes containingcomponents as described before were used, which were collected by anelectrostatic precipitator disposed in a fire flue of a boiler takingplace combustion of C heavy oil while adding ammonia to the fire flue.

(1) Preparing-step of slurry of combustion ashes

The combustion ashes of petroleum fuels and water were mixed at a 1:4weight ratio, concentrated sulfuric acid was added to adjust the pHvalue to 3, and then the resultant mixture was heated to 50° C. tocontinuously prepare a slurry of combustion ashes.

(2) Metal oxidation step

A closed-type oxidation vessel of 10 m³ volume having a gas supply pipeis disposed so as to protrude in a reaction material (combustion ashslurry), equipped with a heating/temperature retaining device, an innerpressure control device, a cooling condenser device, a liquid supplyport and a liquid discharge port, and having a stirring blade disposednear a location above the opening of the gas supply pipe was used as anoxidation vessel. The opening at the top end of the gas supply pipe hasa doughnut shape with a tube outer diameter of 80 mm and having a numberof vent holes with 4 mm inner diameter, to a circular portion situatedabove.

The slurry of combustion ashes was supplied from the supply port at aratio of 10 m³ /hr by a pressurizing pump under pressure, and theoxidizing reaction was continued while introducing 200 Nm³ of air at atemperature of 50° C. and 0.1 to 0.2 ton/hr of heated steams at 3 atmfrom the opening of the gas supply pipe into the slurry of thecombustion ashes, and vanadium in the slurry of the combustion ashes wasconverted into ammonium metavanadate.

During the oxidizing reaction, in the oxidation vessel, the temperatureof the slurry of combustion ashes was maintained by theheating/temperature retaining device at 125° C., and the internalpressure was maintained to 2.0 kg/cm² G by the internal pressure controldevice. Further, the slurry of the combustion ashes containing ammoniummetavanadate was discharged from the discharge port at the substantiallyidentical rate with the supplied amount, and supply the dischargedslurry to the succeeding step while maintaining the liquid surface to asubstantially identical level. Further, the steams and the ammonia gasdischarged from the valve of the inner pressure control device werecooled by the cooling condenser and refluxed into the oxidation vessel.When the concentration of ammonium metavanadate in the oxidation vesselwas actually measured, it was about 1.5% by weight, and theconcentration of ammonium sulfate was about 15% by weight.

(3) Solid/liquid separation step

While continuously supplying the slurry of the combustion ashescontaining ammonium metavanadate to a pressure proof-type filter andunder temperature retaining, unburnt carbon and iron sludges or the likeby-produced in the metal oxidation step were separated while retainingthe temperature thereof to obtain an aqueous solution containingammonium metavanadate.

(4) Crystallization step for ammonium metavanadate

Into a crystallization vessel of 50 m³ of an inner volume for containing40 m³ of crystallized slurry at a temperature of 35° C., a raw materialsolution at a temperature of 95° C. containing ammonia sulfate at 15 wt%concentration and 4500 ppm of ammonium metavanadate was suppliedcontinuously at a rate of 10 m³ /hr and mixed under stirring. At thesame time, the crystallized slurry was withdrawn from a discharging portof the crystallization vessel at a rate of 10 m³ /hr. Concurrently, amixed crystallized slurry was continuously supplied at a rate of 120 m³/hr to a counter current cooling column, cooled to 30° C. and fed backto the crystallization vessel. The temperature lowering for the slurryin the counter-current cooling column was 5° C.

The concentration of crystals and the dissolving concentration ofammonium metavanadate in the crystallized slurry in the crystallizationvessel were 4410 ppm and 90 ppm respectively, and ammonium metavanadateof 98% by weight in the solution of the raw material was crystallized.

(5) Recovering-step of crystals of ammonium metavanadate

The crystallized slurry withdrawn from the crystallization vessel wasseparated by using a centrifugal filtration device and ammoniummetavanadate was separated by filtration in a cake filtration system,and the obtained filtrate was supplied to a gypsum reactor in thesucceeding double decomposition step.

(6) Double decomposition step for ammonium sulfate

Calcium hydroxide was added to the liquid filtrate supplied in thegypsum reactor to conduct double decomposition of ammonium sulfate,thereby obtaining a slurry containing gypsum, ammonia, nickel hydroxideand magnesium hydroxide. The gypsum concentration in the slurry was 12%by weight and the total amount of nickel hydroxide and magnesiumhydroxide was 3% by weight. Calcium hydroxide was further added to thegypsum slurry to adjust the pH value to 12.5. The gypsum concentrationwas 15% by weight and the concentration of magnesium hydroxide was 3% byweight in the gypsum slurry.

(7) Ammonia recovery step

The gypsum slurry described above was supplied to a counter-currentpacked column, and ammonia was separated and recovered together withsteam. That is, the gypsum slurry, after heated to 80° C., was suppliedfrom the upper portion of the packed column at a rate of 12 m³ /hr.Concurrently, steam at 160° C. was supplied from the lower portionthereof as a separating medium at a rate of 2400 kg/hr. The volume of apacking containment chamber of the counter-current packed column was 1.5m³, and the packings were SUS 303 bell saddles (200 kg). The ammoniacomponent obtained by the separation described above was absorbed in thesteam used as the separating medium, and cooled into aqueous ammonia.

(8) Utilization of ammonia (1)

Aqueous ammonia was transported to an ammonia separation column throughthe transportation pipelines of small inner diameter of 20 mm andammonia was separated. The ammonia separation column was disposed in abattery limit of an ammonia supply device in the downstream of anexhaust channel of a combustion furnace collecting the combustion ashesof petroleum fuels used in the present example, and was present at aplace spaced by 900 m from the position of the packed column. For theseparating procedure, the aqueous ammonia described above was suppliedfrom the upper portion of the separating column and steams at atemperature of 150° C. was supplied as the separating medium from thelower portion of the packed column.

After mixing the heated air with a wet ammonia gas obtained in theammonia separation column (water content: 30 wt %) to form a gaseousmixture (ammonia content 1:1 wt %), and heating to 150° C., theresultant mixture was sprayed from nozzles of the ammonia supply devicedescribed above with the nozzle ports being disposed to the inner wallsurface of the channel downstream to the exhaust gas channel of thecombustion furnaces. In this way, ammonia was utilized as a neutralizingagent for a sulfate gas contained in the exhaustion gases of thecombustion furnace.

(9) Utilization of ammonia (2)

Aqueous ammonia was transported through a fine pipeline of small innerdiameter of 20 mm to an aqueous ammonia storage vessel for the metaloxidation step and supplied from the aqueous ammonia storing vessel tothe metal oxidation vessel.

(10) Gypsum recovery step

A gypsum slurry was supplied continuously at a rate of 14 m³ /hr to aconical horizontal-type continuous decanter and separated into gypsum of10 wt % of water content and a supernatant. The height of the damdisposed to a discharging portion for the supernatant was controlled soas to contain 0.84 wt% of whole hydroxide concentration and 0.06 wt % ofgypsum concentration (corresponding to 7 wt % to the hydroxide convertedas the solid) in the supernatant.

After recovering the supernatant to a tank equipped with a stirrer, itwas supplied to and treated in a flash plate press batchwise. The amountof processing one batch of solids was set to as 60 kg, and the reductionof the filtration rate to about 1/2 was judged as clogging. When theforegoing processing was conducted till the clogging occurred, the flashplate press was clogged at the number of batches of 50 times.

By the way, when the solid/liquid separation was conducted in the samemanner as described above except for mixing hydroxides into gypsum to beseparated in the decanter and changing the whole hydroxide concentrationin the supernatant to 0.4% by weight and the gypsum concentration to0.01% by weight (corresponding to the amount of 0.5 wt % relative to thehydroxides converted as the solid) by elevating the height of damdisposed to the discharging portion of the supernatant, the flash platepress was clogged at the number of batches of 5 times.

Further, as a result of conducting the solid/liquid separation in thesame manner as described above except for decreasing the amount ofgypsum separated in the decanter and changing the whole hydroxideconcentration to 1.0% by weight and the gypsum concentration to 0.2% byweight in the supernatant (corresponding to 20 wt % to the hydroxidesconverted as the solid) by lowering the height of the dam disposed tothe discharge portion of the supernatant.

(11) Evaluation

The operations described above were continued for 330 days and thenstopped.

(a) During the processing for ammonia separation, the clogging for thepacked columns did not occur at all. After the end of the operation, thepackings at the inside thereof were taken out and observed at thesurface. As a result, the deposition of gypsum and magnesium hydroxideon the surface of packings could scarcely be confirmed.

(b) The entire amount of the slurry of the combustion ashes in theoxidation vessel was discharged and the surface of the opening of thegas supply pipe and the surface of the stirring blade were observed. Asa result, the scaling formed by ammonium metavanadate was scarcely foundon both of the surfaces and there was no worry about the clogging of theopening of the gas supply pipe.

(c) The periphery of the cooling surface of the counter-current coolingcolumn in the crystallization step was observed. As a result, there wasno scaling of the crystals of ammonium metavanadate and the coolingperformance was not substantially lowered.

Reference Example 1

Stripping for free ammonia was conducted in the same manner as inExample 1 except for changing the pH value of the gypsum slurry suppliedto the ammonia recovery step to 10. When the ammonia stripping wascontinued for 60 days, since the slurry began to be over flowed due tothe clogging in the packed column, the processing was interrupted. Then,the slurry stagnated in the inside of the packed column was removed andthe inside thereof was observed. As a result, a large amount ofmagnesium hydroxide was deposited on the surface of the packings.

Reference Example 2

Stripping for free ammonia was conducted in the same manner as inExample 1 except for changing the gypsum concentration in the gypsumslurry supplied to the ammonia recovery step to 5% by weight. When theammonia stripping was continued for 50 days, since the slurry began tobe over flowed due to the clogging in the packed column. Then, theprocessing was interrupted and the slurry stagnated in the inside of thepacked column was removed and the inside thereof was observed. As aresult, a large amount of gypsum was deposited on the surface of thepackings.

What is claimed is:
 1. A method for the recovery of products from the combustion ashes of petroleum fuels, collected by a dust collector disposed in an exhaust gas fire flue of a boiler using petroleum fuels, wherein the combustion ashes contain ammonium sulfate, said method comprisingpreparing a slurry by mixing water with said combustion ashes; injecting either calcium oxide or calcium hydroxide into said slurry and reacting the ammonium sulfate with either calcium oxide or calcium hydroxide to form gypsum and ammonia; recovering ammonia from the gypsum slurry by flowing said gypsum slurry down from an upper portion of a packed column and blowing air or steam from a lower portion of the packed column to bring the air or steam into counter-current contact with said gypsum slurry and thereby stripping ammonia therefrom; and separating gypsum from said gypsum slurry.
 2. A method according to claim 1, wherein the solid concentration of gypsum in the gypsum slurry is 7 to 40% by weight.
 3. A method according to claim 1, wherein the pH value of the gypsum slurry is not less than
 11. 4. A method according to claim 1, wherein two units of solid/liquid separators are used in series in the gypsum separation step, the first solid-liquid separator being a centrifugal precipitation-type solid/liquid separator and the second solid/liquid separator being a centrifugal precipitation-type separator or a filtration type solid/liquid separator, said step involvingprocessing said gypsum slurry in said first solid/liquid separator to separate gypsum from a supernatant containing (a) nickel and/or magnesium hydroxide and (b) 1 to 20 weight percent of gypsum, calculated as solids, based on said hydroxides, and processing said supernatant in said second solid/liquid separator to form a solid and waste water. 